1. Field of the Invention
The present invention relates generally to monolithic microwave integrated circuit (MMIC) design, and in particular, to a broadband, 4-bit MMIC phase shifter for use in a phased array antenna employed in radar or communication systems to point or steer an RF (Radio Frequency) beam.
2. Description of the Related Art
In the prior art, a phase shifter is employed as one of the component circuits of a phased array antenna and the like. FIG. 1 is a block diagram schematically showing the configuration of a prior art phased array antenna. In the figure, a phased array antenna 200 includes a plurality of antenna elements 211, 212, 213, and 214. The antenna 200 changes the direction D of an incoming or outgoing electromagnetic wave by controlling the phase of the electromagnetic waves in the antenna elements 211, 212, 213, and 214. FIG. 1 shows a phased array antenna which has four antenna elements for simplicity of description. Typically, there are more than four antenna elements in an actual phased array antenna.
The antenna 200 includes amplifiers 221, 222, 223, and 224, all of which amplify microwaves going out from or coming into the corresponding antenna elements 211, 212, 213, and 214, and phase circuits 231, 232, 233, and 234, all of which shift the phases of microwaves going out from or coming into the corresponding antenna elements 211, 212, 213, and 214. The phase circuits 231, 232, 233, and 234 are connected to a signal source 260 and a signal receiver 270 via corresponding directional couplers 251, 252, 253, and 254.
The antenna 200 also includes a control circuit 240 which controls the phase circuits and the directional couplers. More specifically, the control circuit 240 controls the phase shift of the phase circuits 231, 232, 233, and 234 with 5-bit control signals Pc1, Pc2, Pc3, and Pc4, respectively, and switches the connection of each phase circuit to the signal source 260 or to the signal receiver 270 with a control signal Kc.
FIG. 2 shows the specific configuration of the phase circuit having input and output terminals 23a and 23b, respectively. As shown in FIG. 2, each of the phase circuits 231, 232, 233, and 234 of FIG. 1 comprises five switched-line phase shifters 230a, 230b, 230c, 230d, and 230e, all of which provide different phase shifts. The phase shift is defined as the difference in phase between signals at the phase shifter output and input. In the phase circuit so constructed, the phase of microwave input can be varied in steps of 11.25xc2x0 in the range of from 11.25xc2x0 to 348.75xc2x0 using a 5-bit control signal.
Referring back to FIG. 1, the traveling direction of the microwaves radiated by the antenna 200 is a direction D that is perpendicular to a wavefront W. The wavefront W consists of parts having the same phase in the microwave signals radiated from the antenna elements. In other words, microwaves are radiated from the antenna 200 in the direction D. The radiation direction D depends on the phase shift set by the control signals Pc1, Pc2, Pc3, and Pc4 in the phase circuits 231, 232, 233, and 234.
Increasing carrier frequencies in communications systems offer greater data transmission rates. Changing from X-band (8 GHz) to Ka-band (32 GHz) has the potential for a sixteen fold improvement in data rates for a given antenna size and transmitter power. In deep space missions, where the available DC power is limited, improving the amount of data returned in a given mission is of great interest.
The phase shifter is a key component in a phased array system. However, designing at Ka-band frequencies, especially regarding phase, is much harder than at lower frequencies. An error in line length amounting to 5xc2x0 at X-band (8 GHz) becomes a 20xc2x0 error at Ka-band (32 GHz). Modeling errors in switch devices, microstrip or MMIC components accumulate quickly. Conventional switching devices are far from ideal at Ka-band.
Additional factors must be considered when designing high frequency phase shifters. For example, design trade-offs for insertion loss, insertion balance between phase states and phase accuracy must be made. Further, each bit of the phase shifter uses a topology or circuit architecture appropriate for that particular phase shift, however, a topology that works well for large phase shifts may be inappropriate or inefficient for small phase shifts. Additionally, parasitic capacitances in the switches must be compensated for, or incorporated into, the phase shifter topology.
It is, therefore, an object of the present invention to provide a broadband (Ka-band) MMIC phase shifter.
It is another object of the present invention to provide a broadband phase shifter which reduces the effect of parasitic capacitances in the switching elements.
The foregoing and other objects of the present invention are achieved by providing a broadband, 4-bit MMIC phase shifter. Rather than use exotic GaAs processing techniques to reduce the effects of parasitic capacitances in the switch elements, a standard pseudomorphic high electron mobility transistor (PHEMT) process is used, with phase shift architectures chosen to absorb the parasitic effects for broadband operation. Careful analysis of linear and EM simulations in combination with measured results has resulted in an improved four-bit design that is compact, broadband and has good insertion loss and balance, yet uses a standard 0.25 mm PHEMT process with standard bias voltages.
The four-bit selectable phase shifter for use in a phased array antenna of the present invention, which selectably causes an input signal to be shifted in phase, includes a first bit for selectively providing a 180xc2x0 phase shift, wherein the first bit is a line/reflected bit; a second bit for selectively providing a 90xc2x0 phase shift, wherein the second bit is a reflected bit; a third bit for selectively providing a 45xc2x0 phase shift, wherein the third bit is a reflected bit; and a fourth bit for selectively providing a 22.5xc2x0 phase shift, wherein the fourth bit is a high pass/low pass bit.